Automatic question answering (QA) is a form of information retrieval in which focused answers are generated for either user queries, e.g., a key word search, or ad hoc questions, e.g., questions in a natural language format. A question answering system can attempt to handle various question types including: fact, list, definition, how, why, hypothetical, semantically constrained, and cross-lingual questions. The questions can be either in a closed domain or open domain. Closed-domain questions are under a specific knowledge domain (e.g., medicine or physics). Open-domain questions can relate to any topics and usually rely on general knowledge for answers. Question answering is regarded as requiring more complex natural language processing (NLP) techniques than other types of information retrieval such as document retrieval.